proof that bash can differentiate between undefined and an empty string

readme.md

Example output

$ ./undefined_vs_empty.bash
x was not set.
$ ./undefined_vs_empty.bash -x ''
x was set to: ''
$ ./undefined_vs_empty.bash -x hello
x was set to: 'hello'

undefined_vs_empty.bash

#! /usr/bin/env bash

# try it:
# $ ./undefined_vs_empty.bash
# $ ./undefined_vs_empty.bash -x ''
# $ ./undefined_vs_empty.bash -x hello

parse_cli() {
  local OPTARG OPTIND
  local x
  
  while getopts ':x:' opt "$@"; do
    case "$opt" in
      x)
        x="$OPTARG"
        ;;
    esac
  done
  shift "$(( OPTIND - 1 ))"
  
  if [[ "${x+defined}" = 'defined' ]]; then
    # if $x is defined, ${x+defined} will substitute in 'defined'
    # this will take empty strings into account and treat them as defined.
    echo "x was set to: '$x'"
  else
    # $x is undefined
    echo "x was not set."
  fi
}

parse_cli "$@"